that will be used in systems external to the process control system. This can be key performance indicators (KPIs) or efficiency values such as energy per unit volume treated, chemical user per unit treated, or wire-to-water pump efficiency, or data used in a computerized works management system such as motor run times, bearing temperatures, or vibration for predictive driven maintenance. All calculations should be listed for integration in the controller or supervisory control system. Process data points that require static trending and generation of automatic reports should be identified for implementation by the programmer.
3.0 OTHER METHODS OF CONVEYING PROCESS DESCRIPTIONS
It can be difficult to describe a treatment process or facility so everyone understands the designer’s or facility manager’s intent. Many larger, older treatment plants have been expanded numerous times, and interconnections between expansion phases can be difficult to follow. In addition, people who do not work in the wastewater treatment industry may not immediately understand the underlying biochemistry or chemistry of each treatment process.
To avoid losing readers, process descriptions should begin simply, laying out the “big picture”, and then become progressively more detailed. Tools for, and examples of, simplifying delivery of process and control information are identified in this section.
3.1 Animations
Animations help eliminate confusion and clearly depict how a particular process or facility is intended to work. Animations of various treatment processes (e.g., conventional activated sludge systems, sequencing batch reactors, five-stage biological nutrient removal processes, and complex hydraulic systems) help people better understand them.
Given tools currently available to digital graphic artists, animation development is relatively straightforward and inexpensive. Effort and costs are justified by the ease with which viewers understand project design.
Animation can also be invaluable during the construction phase. The transportation industry often uses animation at public meetings to illustrate a project at various stages of construction; as a result, attendees typically feel much better informed about a project. These animations can be linked to the PCN as hypertext, if the animation file is present on a shared network drive or intranet, or linked directly onto an HMI display for immediate access by operators.
3.2 Static Process Models
Static process models are valuable teaching tools in that they allow users to change one process variable and see how it affects process results. To be effective, the interface should be user friendly and the format should enable the user to easily see the process responding to changes. Successful models typically are relatively simple spreadsheets, frequently developed in Microsoft Excel. Basically, if one can draw a graph of the process, he or she can develop a useful model.
These models have been developed for many applications (e.g., chemical-feed controls; anaerobic- or aerobic-digester loads; and sludge age, mean cell residence time, and food-to-microorganism ratio calculations).
3.3 Interactive Process Models
Interactive process models allow users to become familiar with the rate at which a process change occurs. These models are typically spreadsheets with interactive bar charts (sliders) displaying the important process variables. Users can change any process parameter and watch other variables respond. The basic difference between interactive and static models is the “real time” response to process variable changes.
4.0 DEVELOPMENT, UPDATES, AND PRESENTATION
A PCN is a living document and requires regular annual reviews and updates as the software, process equipment, or standard operating procedure is changed. Management of this change process is described in this section.
4.1 Document Development and Updates
For most facilities, PCNs are available from a previous process control software upgrade or a project that expanded or built the process or facility. This provides a starting point for reviewing and updating the PCN for a new project that will change the process or software. If a PCN is not available, the process description or a document from a similar process or facility can be used as a basis for the PCN.
During a change in the software or process, the PCN will be updated in parallel to the P&IDs as the process is defined and new monitoring and control elements are introduced. During this process, flow charts, equipment operation tables, and control descriptions will undergo many iterations. Once these changes are complete, alarms, setpoints, and performance data should be finalized.
Following a change to the process or upgrade to the software, changes may be made to the operating parameters of the system. These changes to alarm limits, setpoints, and KPIs should be updated in PCNs to keep the document up to date as an operator reference, training tool, and base document for future upgrades.
4.2 Document Storage and Presentation
Because the PCN is a design document, many edits and updates will be made during its development. Upon acceptance of the software and the process, the document should be stored electronically and protected. This will allow the document to be used by operators and staff while edits are controlled. As permanent changes are identified in the process and implemented in the software, these changes must be reflected in the PCN and implemented in a secure manner.
If a document management system is in use, the PCN can be stored, protected, and accessed from that system. If this is not available, a secured drive with a regular backup program is suitable. A link on an HMI screen could be used to allow operators direct access to the PCN from the associated process page or a link through the intranet. Relying on printed materials may result in superseded versions of the PCN being used, copied, or distributed, thereby providing incorrect information.
5.0 REFERENCE
American National Standards Institute and International Society of Automation (2009) Instrumentation Symbols and Identification Standard (ANSI/ISA-5.1-2009); International Society of Automation: Research Triangle Park, North Carolina.
Chapter 6
Specifications
Thomas H. Powell, P.E., C.C.S.
Mohamad Bassidgi, P. Eng
1.0 RELATIONSHIP OF SPECIFICATIONS AND DRAWINGS
2.1 Standard Specifications
2.2 Project-Specific Specifications
2.3 Vendor-Supplied and -Suggested Specifications
